In recent years, the demand for broadband and high-speed communication services has grown and the development of radio communications systems is being advanced to implement these communication services. Examples of broadband and high-speed communication services include best-effort data communication, VoIP voice communication, streaming delivery of image contents, and the like.
A cellular radio communications system of the third generation using CDMA (Code Division Multiple Access) is capable of providing users with multimedia information through an IP network.
A cellular radio communications system using OFDM (Orthogonal Frequency Division Multiplexing) has attracted attention as a next-generation CDMA radio communications system. OFDMA is a technology in which multiple orthogonal carrier waves are subjected to quadrature modulation on a frequency axis to enhance the utilization efficiency of frequency.
The cellular radio communications system using OFDMA is a cellular radio communications system of the 3.9 generation. Typical standards of OFDMA are LTE (Long Term Evolution) and UMB (Ultra Mobile Broadband). These standards are respectively internationally set by the 3GPP (3rd Generation Partnership Project) and the 3GPP2.
It is known that the cellular radio communications system using OFDMA is more susceptible to interference power from an adjacent base station than the cellular radio communications system using CDMA is. For this reason, with respect to the cellular radio communications system using OFDMA, it is difficult to work out a design for the disposition of base stations. Especially, at the end of the cell (cell edge) of a relevant base station, the power level of a radio signal transmitted from the relevant base station and the power level of interference power from an adjacent base station get close to each other. This causes a problem of markedly degraded SINR (Signal to Interference and Noise power Ratio), which is an index indicating channel quality.
To solve this problem, it is advisable to adopt FFR (Fractional Frequency Reuse) in a cellular radio communications system. FFR is a technology for allocating different frequency bands for which high power is set to a mobile station positioned at the cell edge of some base station and a mobile station positioned at the cell edge of another base station adjoining thereto. According to FFR, a cellular radio communications system can reduce the influence of interference power from an adjacent cell on a mobile station positioned at each cell edge.
FIG. 2A is an explanatory drawing illustrating an overview of a conventional non-FFR cellular radio communications system.
The cellular radio communications system that does not adopt the FFR transmits radio signals by identical power using the full band (f0) of a frequency resource. For this reason, it is possible to schedule the transmission and reception of radio signals using an identical frequency band with respect both to the central portion of a cell (cell center) and the cell edge. However, a mobile station positioned at the cell edge of a base station 201 is largely influenced by the interference power of a radio signal transmitted from another base station 201 adjoining thereto.
Consequently, it was proposed to adopt the FFR in a cellular radio communications system.
FIG. 2B is an explanatory drawing illustrating an overview of a conventional FFR cellular radio communications system.
The cellular radio communications system that adopts the FFR divides a frequency band (f0) in which radio signals are to be transmitted into multiple frequency bands and sets powers different in magnitude for the divided frequency bands (for example, f1, f2, f3). The cellular radio communications system varies the combination of the divided frequency bands f1, f2, f3 and powers set for the frequency bands f1, f2, f3 on a base station 201-by-base station 201 basis. The cellular radio communications system can thereby reduce interference power a mobile station positioned at the cell edge of a base station 201 receives from an adjacent cell.
FIG. 2C is an explanatory drawing illustrating frequency bands in conventional FFR.
The frequency band (f0) in which radio signals are to be transmitted is divided into multiple frequency bands (for example, f1, f2, f3).
One of technologies for accelerating data communication is HARQ (Hybrid Automatic Repeat reQuest). HARQ is a technology for retransmitting packets implemented in the physical layer and the MAC (Media Access Control) layer. HARQ is superior to other retransmission technologies implemented in the RLC (Radio Link Control) layer. In HARQ, there are two synthesis methods, IR (Incremental Redundancy) and CC (Chase Combining).
In CC, the transmitting side transmits a packet more than once and the receiving side synthesizes data contained in the packets for the multiple times. According to CC, power (reception level) is enhanced as the number of times of retransmission is increased; therefore, the probability of success in decoding data is increased.
In IR, meanwhile, the transmitting side retransmits packets containing an error correcting code more than once; and the receiving side decodes data contained in the packets using each error correcting code contained in the received packets for the multiple times. According to IR, the following takes place as the number of times of retransmission is increased: in addition to the power synthesis effect which is an effect of CC, the redundancy bits of the error correcting codes used by the receiving side to decode data are increased. Therefore, the probability of success in decoding data is increased. Both in IR and in CC, therefore, the following takes place when the number of times of packet retransmission is increased: the probability of success in decoding data on the receiving side is increased and thus the probability of transmission success (ACK) is increased.
FIG. 3 is an explanatory drawing illustrating an overview of conventional HARQ.
In HARQ, power is divided based on a target number of times of retransmission. The target number of times of retransmission cited here refers to the number of times of retransmission of traffic required for meeting a power requirement. When the target number of times of retransmission is one as illustrated in FIG. 3 as an example, power for one time of transmission is set high. Therefore, the added power exceeds the required power by transmission of the second traffic. When the target number of times of retransmission is three as illustrated in FIG. 3, power for one time of transmission is set low. Therefore, the added power exceeds the required power by transmission of the fourth traffic.
To maximize the effect of HARQ, in general, it is advisable to increase the target number of times of retransmission. In HARQ, MCS (Modulation and Coding Scheme) is selected so that the required power is exceeded at the target number of times of retransmission.
In HARQ, as mentioned above, the probability that transmission will succeed (cumulative success probability) is enhanced as the target number of times of retransmission is increased. When the target number of times of retransmission is large, transmission may succeed before the original target number of times of retransmission is not reached (early termination).
However, when the target number of times of retransmission is three as illustrated in FIG. 3 as an example, there is a problem of lengthened delay time in communication between the transmitting side and the receiving side. Therefore, with respect to traffic on which a strict delay requirement is imposed, the target number of times of retransmission cannot be increased. For this reason, the following measure is taken in cellular radio communications systems using the existing CDMA to reduce the target number of times of retransmission: high power is set for traffic on which a strict delay requirement is imposed.
When high power is set for retransmission traffic, however, a problem arises. As illustrated in the case where the target number of times of retransmission is one illustrated in FIG. 3, power added by retransmission largely exceeds a required power and the power becomes excessive.
In HARQ, the retransmitted packet is a packet used for reducing an error rate. Therefore, the reception quality of retransmitted packets received on the mobile station side may be lower than the reception quality of the initially transmitted packet. For this reason, with respect to the cellular radio communications system using the existing CDMA, it is proposed to reduce the power of retransmitted packets. (Refer to JP-A-2004-173017, for example.) In JP-A-2004-173017, there is the following description: “In the IR-type HARQ, the second and following packet transmissions are auxiliary and do not require large power; therefore, power for the second and following times is reduced before transmission.”